Membranes play a crucial role in almost all cellular phenomena, yet we know little about the molecular organization of these structures or the functions which they carry out. The lipid component of many natural membranes has been well characterized, but we know little about the protein component mainly because of the insoluble nature of these proteins which makes them difficult to purify and study. However, it is only the protein component of membranes which can confer the high degree of specificity necessary to carry out the complex functions associated with biological membranes. In addition, proteins must play an important role in membrane structure. Methodology is now available for purification of membrane enzymes in an active form which should be a good criterion for retention of native structure. The enzymes of phospholipid metabolism are good proteins to begin purifying because study of these pure enzymes should not only shed light on membrane structure and function, but should also lead to a better understanding of phospholipid metabolism and control, as well as the coordination of lipid and protein synthesis with membrane biosynthesis.